In a typical electrophotographic machine (e.g. copier, duplicator, printer, etc.), a continuous loop of a photoconductor film is commonly used to transfer an image from an input section onto a copy medium (e.g. a sheet of paper or the like). The film is initially charged and passed through an input section where an image is projected onto the charged film. The film then moves through a developing section where toner is applied to the charged image, and on through an image transfer section where the toner is transferred to a sheet of paper or some other medium. The toner (i.e. image) is then fixed (i.e. fused) to the sheet by passing the sheet between a pressure roller and a heated roller within the fuser section of the machine.
In typical electrophotographic machines of this type, it is common to use a vacuum transport to transfer the sheet from the film loop to the fuser section. Often this vacuum transport is directly interfaced between the film and the fuser section wherein the vacuum transport receives the sheet from the film and passes it directly into nip between the rollers in the fuser section. This requires that the surface speeds of (a) the film loop, (b) the vacuum transport belt(s), and (c) the fuser rollers all have to be closely matched. If the speeds become mismatched, there may be relative movement between the film and the sheet while the image is being transferred onto the sheet thereby resulting in smearing of the image on the sheet.
When such relative movement occurs, it is normally at the trail edge of the sheet as the trail edge passes between the image transfer roller and the detack roller since, at this point, the trail edge is held in contact with the film only by electrostatic forces. The speed of the sheet, itself, is controlled by either the speed of the vacuum transport or the speed of the rollers in the fuser section depending on the length of the respective sheet. By matching the vacuum transport belt speed to the film speed, normally there will be no relative movement between the sheet and the film if the length of the sheet is shorter than the straight-line distance between the detack point and the fuser rollers.
However, for sheets longer than this straight-line distance, the sheet will be engaged by the nip between the fuser rollers while the trail edge of the sheet is still engaged between the transfer roller and the detack roller. If this happens, the sheet will move relative to the film and smearing will almost always occur. Therefore, it is imperative that the speeds of these components be synchronized and maintained throughout the entire copy operation.
Unfortunately, due to the dynamic nip mechanics present in all compliant fuser roller(s), the real possibility exists that the speed of the film in the image transfer section and that of the fuser rollers will become mismatched at some time during the copying operation. If and when this occurs, relative movement occurs and smearing appears at the trail edge of the sheet.
To alleviate this problem, some commercial machines have now abandoned any direct interface between the film and the fuser section and instead, use a curved or arched travel path between the image transfer and the fuser sections which is longer than the straight-line distance between these sections (i.e. curved path is longer than the length of any sheet to be used in the copy operations). This extended path effectively “de-couples” the speed of the fuser rollers from the speed of the film thereby eliminating the possibility of relative movement between the sheet and the film.
That is, since the trail edge of any standard sheet used in the machine, up to the maximum length designed for (e.g. up to 17 inches), will be clear of the detack roller before any slack in the sheet is taken out by the fuser rollers, this extended travel path creates a “buffer zone” between the image transfer section and the fuser section. Accordingly, if a speed mismatch should occur, the trail edge of a sheet will be clear of the detack roller before any “overdrive” in a sheet is taken up by the fuser rollers.
Typically, such an extended, curved travel path is provided by angling the vacuum transport away from the straight-line distance between the sections and then positioning a fuser entrance guide between the exit end of the vacuum transport and the entrance of the fuser section. The fuser guide is normally vacuum assisted along the edges thereof so that the sheet is held against the guide to orient the sheet as it enters the fuser section. This type of curved travel path and guides are known and has been commercially used, e.g. DIGIMASTER 9110, Heidelberg Digital L.L.C., Rochester, N.Y.
Where extended travel paths of this type are used, it is particularly important to prevent the trail edge of the sheet from falling away from the fuser entrance guide as the trail edge moves into the fuser section. If this happens, the unfused image on the moving sheet may contact other elements in the paper path before it enters the fuser section thereby again raising the possibility that smearing may occur. In known prior art guides of this type, vacuum ports are provided only near the outer edges of the guide's lower surface. Vacuum force through these ports hold the edges of the sheet against the surface as the sheet moves into the fuser. However, since there is no vacuum being applied against the center of the sheet, the possibility exists that the trailing edge and/or the central portion or the trail edge of the sheet may still sag or droop and contact other elements in the paper path thereby causing smearing at point(s) of contact.
Accordingly, it is highly desirable that the fuser entrance guide be able to hold the entire sheet against the guide without sagging until the sheet has passed completely into the fuser section.